The invention relates generally to an apparatus for continuous hot tinning of printed circuit boards, and a process for soldering the terminals of components to the conductor tracks and soldering eyes of a printed circuit board.
The manufacture of printed circuit boards or cards is increasingly changing over from using galvanic processes to a hot tinning process. In such a process, the copper conductor tracks which are already applied to an insulating base material or substrate are coated with a eutectic lead-tin layer.
For the sake of simplicity herein, reference is only made hereinafter to a tin layer and a hot tinning operation.
Hot tinning operations include a cyclic process which operates in a vertical mode and a continuous process which operates in a horizontal mode. In the cycline process, the printed circuit boards or cards are dipped into liquid tin. When the printed circuit boards are taken out of the tin, they are then passed between nozzles and hot air is blown thereonto, thereby blowing free the holes and bores in the board while also blowing away excess tin so as to make the layer of tin applied thereto move even and flat. On the other hand, in the horizontal-mode continuous process, the board is guided through a standing wave of solder or tin material which is produced in front of baffle plates of a nozzle out of which the molten tin is discharged. The printed circuit board is then passed between two nozzles and blown off using hot air.
Prior to the tinning operation, the printed circuit board is treated with a fluxing agent. However, a part of fluxing agent tends to remain clinging to the board and is thus carried into the tin. The fluxing agent burns in the hot tin, with the result that in a tin bath which is not stirred or agitated in some suitable fashion, the residues of the burnt fluxing agent float on the surface of the tin. By virtue of the continuous movement in which the bath is involved in the course of operation however, such residues are entrained with the tin and thus contaminate the surface of the circuit board. Such residues also have a tendency to clog the conduits, lines and pumps of the system. Finally, they also put the surfaces of the conveyor rollers used in the system under additional stress so that the conveyor roller surfaces tend to suffer from a high rate of wear. All that means that extensive cleaning and maintenance operations are required during each working shift involved in carrying out the hot tinning process.
Furthermore, as already indicated above, after the actual hot tinning operation, the printed circuit boards are blown off using hot air, and the fact that the air has to be suitably heated involves the consumption of a great deal of heat energy and thus increases operating costs.
To give a more detailed picture of the kind of equipment involved, reference may be made to U.S. Pat. No. 2,821,959 diclosing an apparatus for soldering components to a printed circuit board or card, comprising a printed circuit board heating station, a fluxing station, a hot tinning station, a cleaning station and a drying station, and further including conveyor rollers which are arranged horizontally above and below the path of conveying movement of the printed circuit boards, for engaging the printed circuit boards to convey them through the equipment. The apparatus further has a trough at the hot tinning station, for accommodating the molten tin, at least one nozzle for applying the tin to the copper conductor tracks on the printed circuit board, with a conduit between the trough and the nozzle, the conduit going upwardly beyond the plane of the nozzle. A pump is provided for conveying the tin from the trough through the conduit into the nozzle, while the tin in the trough is heated by suitable heating means.